IL-12 is a key promoter of cellular immune responses that has been shown to be therapeutically active against many murine tumors when used alone, or as a vaccine adjuvant. Use of rmIL-12 during vaccination with irradiated, genetically engineered SCK murine mammary tumor lines was shown recently to enhance vaccine efficacy but only after periods of immune suppression that severely compromised host tumor protection. Suppression may be a general effect of IL-12 and may be linked to transiently impaired splenic T cell mitogenesis responses. These observations suggest that non-immunological mechanisms activated by rmIL-12 promote tumor regression during the period of immune suppression. IFNg, which is secreted in response to IL-12, is required for these other mechanisms. IFNg clearly activates important host anti-tumor effects such as angiogenesis inhibition, but its effects on tumor cells are also important as shown by the refractoriness to rmIL-12 therapy of SCK cells rendered IFNg-un-responsive by expression of a dominant negative IFNgR. Studies in this application will examine mechanisms of tumor rejection activated by rmIL-12 and attempt to put them in an overall therapeutic perspective. Hypotheses to be tested are that (a) rmIL-12 enhances anti-tumor immunity but has immune suppressive effects the extent and cause of which need to be determined; (b) other tumoristatic or tumoricidal mechanisms activated by rmIL-12 through IFNg, such as angiogenesis inhibition, account for tumor regression during the period of immune suppression; and (c) specific genes or effectors activated by IFNg in tumor cells determine their susceptibility to rmIL-12 antitumor mechanisms. Specifically, Aim 1 will characterize conditions of rmIL-12 use that suppress and later enhance tumor vaccination and will define the enhancing and suppressive effects of reIL-12 on new and mature responses to tumor and allogeneic immunization. Aim 2 will examine the role of IFNg and TNFa as mediators of suppression, and Aim 3 will examine the contribution of angiogenesis inhibition and other anti-tumor mechanisms during the rmIL-12 treatment of growing tumors to explain how regression is induced despite its immune suppressive effects. Aim 4 will examine the susceptibility of tumor cells made unresponsive to IFNg by a dominant negative IFNgR to specific rmIL-12 anti-tumor mechanisms and will examine the role of candidate IFNg modulated genes in determining tumor cell susceptibility to rmIL-12.